1. Whenever a bond is formed (of whatever kind), energy is released; that makes the things involved more stable than they were before. The more bonds an atom can form, the more energy is released and the more stable the system becomes. In the chlorine case, each chlorine atom only has one electron to share, so it can only form one covalent bond. The Cl 2 molecule is much more stable than two separate chlorine atoms.

2. What is the arrangement of the electrons around each chlorine atom in Cl 2 ? 2, 8, 8 This is the same as the electron configuration of an atom of which noble gas? Argon

3. When atoms bond covalently, they often produce outer electronic structures the same as noble gases – in other words, with four pairs of electrons (or one pair in the case of helium). There are, however, lots of examples where different numbers of pairs are formed, producing structures which are quite unlike noble gases.

4. Hydrogen, a non-metal with one electron in its outer electron shell, needs one electron to complete its outer electron shell. Oxygen, a non-metal with six electrons in its outer electron shell, needs two electrons to complete its outer shell. Question: How many hydrogen and oxygen atoms will be covalently bonded together?

5. Two hydrogen atoms are needed to provide the two electrons required by oxygen. Each hydrogen atom shares one electron pair with an oxygen atom.

6. The formula for water is H 2 O.

7. Carbon is a non-metal with four electrons in its outer electron shell. Carbon needs four electrons to complete its outer electron shell. Hydrogen is a non-metal with one electron in its outer electron shell. Hydrogen needs on electron to complete its outer electron shell.

8. Questions 1. How many hydrogen and carbon atoms will be covalently bonded together in a methane molecule? 2. Draw the electronic configuration of a methane molecule.

10. Oxygen needs two electrons to have a completed outer shell. Questions 1. How many oxygen atoms will be covalently bonded together in an oxygen molecule? 2. Draw the electronic configuration of an oxygen molecule.

11. Answer Two oxygen atoms come close enough for their outer electron shells to overlap. The two atoms share two electron pairs between them, forming a double covalent bond.

12. Answer The formula for oxygen gas is O 2. The double covalent bond is represented as O=O.

13. Carbon needs four electrons and oxygen needs two electrons to complete their outer electron shells. Questions 1. How many carbon atoms will be covalently bonded to oxygen atoms in a carbon dioxide molecule? 2. Draw the electronic configuration of a carbon dioxide molecule.

14. Two oxygen atoms are needed to provide the four electrons required by carbon. Each oxygen atom shares two electron pairs with a carbon atom, forming a double covalent bond.

15. The formula for carbon dioxide is CO 2. The double covalent bonds are represented as O=C=O

16. Nitrogen in Group 5, needs three electrons to complete its outer electron shell. Questions 1. How many nitrogen atoms will be covalently bonded to each other in a nitrogen molecule? 2. Draw the electronic configuration of a nitrogen molecule.

17. Two nitrogen atoms come close enough for their outer electron shells to overlap. The two atoms share three electron pairs between them, forming a triple covalent bond.

18. The formula for nitrogen gas is N 2. The triple covalent bond is represented as

19. The drawing of dot-and-cross diagrams to represent covalent bonding can be simplified by indicating only the electrons which are involved in bonding, i.e. only drawing the electrons in the outermost shell.

20. The formula of a covalent compound gives the exact number of atoms per molecule. In some covalent compounds the molecular formula and the ratio of the elements are the same, e.g. water (H 2 O), carbon dioxide (CO 2 ) and ammonia (NH 3 ). In others they are not the same, e.g. the molecular formula of glucose is C 6 H 12 O 6, and ethane is C 2 H 6.

21. The names of covalent compounds can sometimes indicate the ratio in which the atoms combine with each other. The prefixes di, tri, tetr and so on are used in this kind of naming. PrefixNumber of Atoms mono1 di2 tri3 tetr4 pent5 hex6

22. Examples carbon monoxide (CO) sulphur dioxide (SO 2 ) sulphur trioxide (SO 3 ) carbon tetrachloride (CCl 4 ) dinitrogen tetroxide (N 2 O 4 ) It is best to learn the molecular formulae of covalent compounds as you encounter them. PrefixNumber of Atoms mono1 di2 tri3 tetr4 pent5 hex6

23. In this lesson we learnt about: Definitions and appropriate examples of atoms, molecules and compounds. Dalton’s atomic theory; including contributions by Rutherford, Moseley, Thompson and Chadwick. The properties of protons, neutrons & electrons.

24. The concept of isotopes and their relative abundance with reference to chlorine, carbon & oxygen. The difference of atoms of elements: examples including transition elements. The Octet rule. Ionic and covalent bonding.